Aircraft fuel metering systems provide the proper amount of fuel to an engine depending on engine and flight conditions (takeoff, cruise, descent, etc.). Current fuel metering systems calculate the amount of fuel needed by the engine and deliver the prescribed amount to the engine where it is combusted. To accommodate the range of fuel delivery, fuel metering systems typically contain a fuel pump, a metering valve for controlling the amount of fuel sent to the engine and a separate differential pressure relief valve for allowing fuel to bypass the engine and ensure that the overall pressure in the fuel metering system is within specifications.
The metering valve and the differential pressure relief valve work together to prevent flow fluctuations. The differential pressure relief valve maintains a precise and nominally constant pressure drop across the metering valve. Thus, the differential pressure relief valve is opened to varying degrees based on the burn rate (i.e. the amount of fuel flowing through the metering valve and sent to the engine). When the metering valve is delivering a large amount of fuel (e.g., takeoff), little fuel needs to flow through the differential pressure relief valve to the bypass. Conversely, when only a small amount of fuel is delivered through the metering valve (e.g., descent), a large amount of fuel needs to flow through the differential pressure relief valve to prevent pressure buildup within the fuel metering system. Wide variation in the amount of bypass flow can be required for a given burn rate. Part of the variation is due to engine design; the fuel pump normally delivers fuel to the fuel metering system based on the engine burn rate. Variation is also due to differences in pumping pressure between new pumps and old, worn or deficient pumps. However, conventional differential pressure relief valves are designed to provide only one pressure solution for a prescribed burn rate. This results in imperfect control of the pressure drop across the metering valve. The pressure relief valve is tasked with providing a “perfect” solution to maintain pressure within the fuel metering system, but based on the number of variables affecting flow through the system, it attempts to provide the perfect solution essentially using a one-size-fits-many approach, ultimately reducing the accuracy of the fuel metering system because the “constant” differential pressure drifts up and down with engine operating conditions.